In electrophotography, an electrophotographic plate containing a photoconductive insulating layer or layers is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the electrophotographic plate to a support such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
An electrophotographic imaging member may be a homogeneous layer of a single material such as vitreous selenium or may be a composite layer containing a photoconductor and another material, or may be several layers. One type of composite imaging material comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. U.S. Pat. No. 4,265,990 discloses a layered photoreceptor having separate photogenerating and charge transport layers. The photogenerating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer. U.S. Pat. No. 5,149,609 filed on Dec. 14, 1990, titled, Novel Polymers for Photoreceptor Overcoating For Use As Protective Layer Against Liquid Xerographic Ink Interaction, discusses the various layers of a multilayered electrophotographic imaging member and is incorporated herein in its entirety by reference.
The charge transport layer of an imaging device may comprise any suitable transparent insulating organic polymer or nonpolymeric material capable of supporting the injection of photogenerated holes and electrons from the charge generating layer and allowing the transport of these holes or electrons through the organic layer to selectively discharge the surface layer.
An especially preferred transport layer employed in multilayer photoconductors comprises from about 25% to about 75% by weight of at least one charge transporting aromatic amine compound, and about 75% to 25% of a polymeric film forming resin in which the aromatic amine is soluble. The charge transport layer is preferably formed from a mixture of an aromatic amine compound of one or more compounds having the general formula: ##STR1## wherein R.sub.1 and R.sub.2 are aromatic groups selected from the group consisting of substituted or unsubstituted phenyl group, naphthyl group, and polyphenyl group and R.sub.1 is selected from the group consisting of a substituted or unsubstituted aryl group, alkyl groups having one to eighteen carbon atoms, and cycloaliphatic compounds having from three to eighteen carbon atoms. The substituents should be free of electron withdrawing groups such as NO.sub.2 groups and CN groups. Typical aromatic amine compounds that are represented by this structural formula include:
i. Triphenyl amines such as: ##STR2##
ii. Bis and polytriarylamines such as: ##STR3##
iii. Bis arylamine ethers such as: ##STR4## and iv. Bis alkyl-arylamines such as: ##STR5##
A preferred aromatic amine compound has the general formula: ##STR6## wherein R.sub.1 and R.sub.2 are defined above and R.sub.4 is selected from the group consisting of a substituted or unsubstituted biphenyl group, diphenyl ether group, alkyl group having from one to eighteen carbon atoms, and cycloaliphatic group having from three to twelve carbon atoms. The substituents should be free of electron withdrawing groups such as NO.sub.2 groups and CN groups.
Examples of charge transporting aromatic amines represented by the structural formula of above include triphenylmethane, bis(4-diethylamine-2-methylphenyl)phenylmethane; 4'-4"-bis(diethylamino)-2'2"-dimethyltriphenylmethane; N,N'-bis(alkylphenyl)-[1,1'-biphenyl]-4,4'-diamine wherein the alkyl is, for example, methyl, ethyl, propyl or N-butyl; N,N'-diphenyl-N,N'-bis(3"methylphenyl)-(1,1'biphenyl)-4,4'-diamine. The amines are dispersed in an inactive resin binder. Molecular weights of the amines can vary from about 20,000 to about 1,500,000.
The preferred electrically inactive resin materials are polycarbonate resins having a molecular weight from about 20,000 to about 120,000, more preferably from about 50,000 to about 100,000. The materials most preferred as the electrically inactive resin materials are poly(4,4'-dipropylidene-diphenylene carbonate) having a molecular weight from about 35,000 to about 40,000, available as Lexan 145 from General Electric Company; poly(4,4'-isopropylidene-diphenylene carbonate) with a molecular weight from about 40,000 to about 45,000, available as Lexan 141 from General Electric Company; a polycarbonate resin having a molecular weight of from about 50,000 to about 100,000, available as Makrolon from Farben Fabricken Bayer A.G.; a polycarbonate resin having a molecular weight of from about 20,000 to about 50,000, available as Merlon from Mobay Chemical Company; polyether carbonates; and 4,4'-cyclohexylidene diphenyl polycarbonate. Methylene chloride solvent is a desirable component of the charge transport layer coating mixture for adequate dissolving of all the components as it has a low boiling point. Other solvents that dissolve these binders include tetrahydrofuran, toluene, trichloroethylene, 1,1,2-trichloroethane, and 1,1,1-trichloroethane.
The charge transport layer of the electrophotographic plate is produced by dissolving the charge transporting material and the polymeric film forming resin in a volatile solvent, applying the solution by any of the usual coating techniques: e.g., spraying, dipping, roll coating, or extrusion, to form a uniform coating on the electrophotographic plate, and evaporating the volatile solvent. The polymeric film forming resin provides a matrix to support the charge transporting material which by itself would be too mechanically weak to be useful on an electrophotographic plate. The solvent provides the means by which the charge transporting material and the polymeric film forming resin can be mixed and formed into a uniform film. Therefore the combined purposes of the polymeric film forming resin and the solvent is to provide the means by which the charge transporting material can be coated and supported to provide a charge carrying path from the charge generating layer to the top surface. It is desirable to find a material that provides the function of the solvent and the polymeric film forming resin, that is, to provide a means by which the charge transporting material can be coated as a uniform layer on the electrophotographic plate and by which the charge transporting material can be made mechanically strong. It is also desirable to find a process by which the generation of volatiles is minimized or eliminated.